According to a conventional communication system, a transmitter specifies a plurality of remote units in a communication network to execute a multicast communication for delivering the same data such as file or dynamic image data. The multicast communication is executed by using a protocol, such as IGMP (internet group management protocol) and MLD (multicast listener discovery). Here, “distribution” is defined as transmitting (and reproducing, if necessary) the data generated by another apparatus in the passage to a plurality of receivers.
When an IP network employing a protocol such as the above IGMP is used, the passage for delivering the multicast packet is established in a direction from a receiver (mobile terminal such as a cell phone or a navigation device) toward a transmitter. Specifically, the receiver transmits an IGMP-join packet (path formation request) to a multicast router to which the receiver is connected (i.e., to a subordinate multicast router). The request for establishing the path includes a group identification data (for example, a group name and a source data) for identifying the multicast group.
Upon receipt of this data, the multicast router registers the requested multicast group of the join packet to its own multicast routing table and transmits a request for establishing a multicast routing path to the multicast router on the transmitter side (i.e., to a superior multicast router). Upon receipt of this request, the multicast router on the transmitter side similarly updates the table, and transmits a request for establishing a multicast routing path. By repeating the above operation, a path is established up to the most significant multicast router.
When a similar join packet to the multicast group is transmitted from a separate receiver, a request is issued to establish a path up to the multicast router of which the path has been established already up to the transmitter. As a result, a path is also established between the receiver and the transmitter.
When the multicast routers have their respective receivers on their respective paths, a multicast packet is distributed (reproduced and transmitted to the paths).
Thus, the multicast router positioned at a branching point where a plurality of multicast distribution paths exist, transmits the multicast packet to only those paths that require the delivery. Therefore, the traffic does not increase unnecessarily, and the band of the network can be effectively utilized.
The IP multicast can be realized by the multicast router which is capable of processing the IP layer (layer 3) which, however, cannot be processed by a data link layer (layer 2) switch. Irrespective of whether the receivers are present on the paths, therefore, the received multicast packet is distributed to all paths inefficiently using the band.
A method called IGMP-snooping is proposed to prevent inefficient use of the band in the layer-2 switch. According to this method, the layer-2 switch receives a join packet (path formation request) from the receiver, makes sure if the receiver is connected to the ports of the layer-2 switch, and delivers the multicast packet to the corresponding ports only to effectively utilize the band.
Here, when the receiver is connected to a relay device such as layer-2 switch via a wire, the relay device must copy the multicast packet by the number of the paths on which the receivers are present and must transmit them. On the other hand, when a plurality of receivers receive the multicast packet via a radio base station, the plurality of receivers are allowed to simultaneously receive electromagnetic wave signals of the multicast packet transmitted by the radio base station. Therefore, the radio base station does not have to copy the multicast packet. Namely, the multicast packet can be delivered to the plurality of receivers using the same band as the one used for transmitting the multicast packet to the one receiver.
As described above, when the receiver is connected to the layer-2 switch via the radio base station, the receiver works as a mobile terminal. The mobile terminal often moves between the radio base stations to which it is connected to switch over the connected radio base stations. In this case, a port of the layer-2 switch to which the previously-connected base station is connected is different from a port of the layer-2 switch to which the radio base station of after having been switched over is connected. To continue the communication while the receiver is moving, therefore, the path for delivering the multicast packet must be switched over in the layer-2 switch before and after the switch over.
Here, the layer-2 switch receives the join packet (path formation request) transmitted from the receiver and recognizes the presence of the receiver ahead of the port and establishes the path. The receiver, however, transmits the join packet only when it takes part in the multicast group and when it receives an IGMP-query packet (request for confirming the presence) from the multicast router.
Therefore, when the receiver which has already taken part in the multicast group moves between the radio base stations, the receiver is no longer allowed to receive the multicast packet until the query packet is transmitted from the multicast router and until the receiver transmits a join packet in response to the query packet. A default value of a transmission interval of query packet of the multicast router is 120 seconds. Therefore, when no receiver taking part in the same multicast group is present under control of the radio base station that is newly connected, it is not allowed to receive the multicast packet for an average of about 60 seconds after it has moved.
Further, when there is no other receiver taking part in the multicast group under control of the previously-connected base station, a query packet is transmitted from the multicast router, and there is no response to the join packet from the radio station of before being switched over. Therefore, the layer-2 switch so recognizes that there is no receiver at the port to which the radio base station is connected.
Therefore, even after the receiver has moved, the multicast packet is delivered from the layer-2 switch to a port to which the previously-connected base station is connected, and the multicast packet is delivered from this radio base station to the radio side. Similarly, therefore, the radio band is inefficiently used an average of about 60 seconds.
In an environment in which the layer-2 switch is connected under control of the multicast router, the radio base station is connected under control of the layer-2 switch, and the receiver is present under control of the radio base station, there is developed a mobile communication system which transmits a join packet (path formation request) by switching over the radio base stations that are to be connected accompanying the motion of the receiver (e.g., US 2004/0213177 corresponding to JP 2004-320725A). According to this mobile communication system, a new delivery path can be established to the layer-2 switch or to the multicast router to shorten the delay time until receiving the multicast packet.
Upon having switched over the radio base station, further, the receiver in this mobile communication system transmits an IGMP-leave packet (“leave” request) to the multicast router to request “leave” from the multicast group. Therefore, the multicast router transmits a query packet and deletes the delivery path depending upon the join packet in response thereto to prevent inefficient use of the radio band in the previously-connected base station.
According to the mobile communication system disclosed in the above publication, however, a leave packet arrives at the multicast router, a query packet in response thereto is transmitted from the multicast router and, further, a join packet must respond to the query packet. These moments inefficiently uses the radio band in the previously-connected base station.